FIELD OF THE INVENTION
This invention relates to an electronic device having a multi layer structure such as a thin film semiconductor device, photovoltaic device, an image forming member for electrophotography, etc., and a method for producing the same.
In the prior art, functional films, i.e., semiconductor thin films for electronic devices such as thin film semiconductor devices, imaging devices, etc., particularly amorphous or polycrystalline semiconductor films, are formed individually by suitable film forming methods from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of an amorphous or polycrystalline, i.e., non single crystalline, silicon film which is optionally compensated for lone pair electrons with a compensating aqent such as hydrogen atoms (H) or halogen atoms (X), etc., (hereinafter abbreviated as "NON Si (H,X)", particularly "A-Si (H,X)" when indicating amorphous silicon and "poly-Si (H,X)" when indicating polycrystalline silicon; the so-called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been employed the vacuum vapor deposition method, the plasma CVD (PCVD) method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been widely used and industrialized.
However, the reaction process during formation of a silicon film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and its reaction mechanism involves not a few unclear points. Also, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rates and flow rate ratio of introduced gases, pressure during formation, high frequency power, electrode structure, structure of reaction vessel, evacuation rate, plasma generating system, etc.). Because of the dependency on such a large number of parameters, the plasma which is formed may sometimes become unstable, which often leads to marked deleterious effects on the deposited film. Besides, such parameters are characteristic of each apparatus and must be selected individually. Therefore under the present situation, it is difficult to standardize the production conditions.
On the other hand, for a silicon film to exhibit sufficiently satisfactory electrical or optical characteristics for particular uses, it is now accepted that the best way to form it is according to the plasma CVD method.
However, depending on the application use of a silicon film, bulk production with reproducibility may be required with satisfactory results in terms of enlargement of area, uniformity of film thickness as well as uniformity of film quality, and therefore in the formation of silicon films according to the plasma CVD method, an enormous investment in the installation is indispensable for a bulk production apparatus and control means for such bulk production is complicated, with tolerance limits of control being narrow and control of apparatus being severe. These are pointed out as problems to be improved in the future.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequencies, microwaves, etc., in a film forming space in which a substrate for film formation thereon is placed, electrons or a number of other ionic species generated therein may damage a film being formed in the film forming process thereby lowering the film quality or causing non uniformity of film quality.
Particularly, in the case of producing an electronic device having a multi layer structure, interface defects formed between the respective layers may cause worsening of the characteristics of the electronic device obtained. There is shown in FIG. 3 an image forming member for electrophotography as an example, which member comprises a substrate 400 made of aluminum, consisting of a charge injection preventing layer (first layer, amorphous silicon doped with boron B) 401, a photosensitive layer (second layer, amorphous silicon not doped with impurities such as B) 402, and a surface protective layer (third layer, amorphous silicon carbide layer) 403 respectively deposited on substrate 400. If all the layers are to be formed by the PCVD method, since the kinds of starting material gases, flow rates and plasma discharging density for formation of the respective deposited layers differ extremely from one another, effort is required to decrease the influence of the interfaces formed between the respective deposited layers, ordinarily by stopping the discharge between the steps for formation of the first layer and the second layer or for formation of the second layer and the third layer in order to completely exchange the gases, or if continuous production is to be employed, by providing a graded layer by varying gradually the kinds of gases, flow rates and plasma discharge intensity, or by providing deposition chambers for formation of the respective deposited layers. In any case, ions generated in the plasma bombard a deposited layer to increase defects. Particularly, in producing an electronic device having a multi-layer constitution, the influence of ion bombardment collision in the plasma is noticeably severe on the interfaces of respective layers. For this reason, satisfactory improvement therein is required.
As described above, in the formation of silicon films, problems to be solved still remain, and it has been desired to develop a method for forming a deposited film which is capable of bulk production with conservation of energy by the use of an apparatus of low cost, while maintaining the characteristics as well as uniformity of the films at a practically applicable level. Particularly, it has been desired to improve the interface characteristics of an electronic device having a multi-layer structure such as a thin film transistor, photovoltaic device, photosensitive member for electrophotography, etc.